POLICE are investigating a serious collision where a young boy was left in a critical condition after being hit  by a car. 

Senior Constable Adam West said police were investigating the circumstances surrounding the crash on Saturday morning, where it's believed a young boy riding his bike was hit by a car on Campbell Street, Newstead.

Police have been told that a seven-year-old boy was struck by a 4WD as he attempted to cross Campbell Street around 11.15am.

The boy was airlifted in a critical condition to the Royal Children's Hospital in Melbourne after 11.30am.

He remained in a critical condition at 5.40pm on Saturday.

The driver of the 4WD, a 51-year-old Newstead man, has been assisting police with their inquires.

Meanwhile, a 50-year-old motorcyclist was injured after he struck a tree in Creswick.

The Magpie man's motorcycle hit a fallen tree on a bush track near Bungaree-Creswick Road in Creswick about 10.40am on Saturday.

He was airlifted from Creswick Oval in a serious but stable condition to The Alfred hospital at 12.30pm with chest, collar bone and rib injuries.